Thermionic Generators
The thermionic generator, a device for converting heat energy to electrical energy, has been known in the art for many years. It utilizes the same principles as the thermionic vacuum tube, an electronic device in which electrons are driven from a cathode to an anode by the application of a high potential bias.
The thermionic generator, a device for converting heat energy to electrical energy, was first proposed by Schlieter in 1915. This device depends on emission of electrons from a heated cathode. In a thermionic generator, the electrons received at the anode flow back to the cathode through an external load, effectively converting the heat energy from the cathode into electrical energy at the anode. Such devices currently show efficiencies of up to 20% for the energy conversion.
This is not high when compared to conventional means for generating electricity. However, there are advantages to using thermionic generators. Heat sources such as solar energy, which is a renewable resource, may be used. Heat energy which would otherwise be a wasted side-effect of an industrial process may be partially and usefully recycled using such devices. Devices may be manufactured using micro-electronic fabrication techniques, for very small generators, where conventional generators are impractical. When compared to conventional devices, such devices are likely to be smaller, weigh less, and cause little or no pollution.
Typically, the space between cathode and anode in such devices must be very small, and there are difficulties in constructing such devices. Vacuum diodes may require spacings of less than 0.001 inch. The spacing can be increased by the use of low pressure diodes with the space filled with a suitable plasma, such as Cesium gas. However, this advantage brings with it further disadvantages, due to the complexity of analyzing the behavior of gases in such an environment, and the heat exchange reactions within the plasma during the operation of the device, which tend to render it less efficient.
Holmlid (U.S. Pat. No. 5,578,886) teaches carbon use to reduce the work function of the collector electrode of a thermionic converter. He suggests that graphite deposited on the foil collector reacts with the collector material to form a carbide. Through interaction between cesium vapor and the carbon-coated surface, high-energy states of cesium are formed, which reduce the work function of the collector and increase the efficiency of the device.
Kennel (U.S. Pat. No. 5,410,166) also teaches graphite use in thermionic converters. Thermionic converters having graphite electrodes have unexpectedly high current densities during reverse bias operation. The n-p junction flooding phenomenon is suggested to be a definitive explanation for these high current densities.
Neither Holmlid nor Kennel teach that graphite use alone is sufficient to reduce work function: the teaching of the former requires the presence of cesium, whilst the teaching of the latter requires reverse bias operation.
In order to encourage the release of electrons from cathode and anode, surfaces of very low work functions must be constructed. Such surfaces have in the past been characterized by the use of very small points, or tips, which have the effect of increasing the potential gradient by concentrating it at the tips, to render electron emission easier.
The current invention addresses these problems in the construction of thermionic generators by applying a known fabrication process, thin film diamond-like-carbon coating, which has not previously been applied to this field. However, before describing the invention, it is necessary to further describe the principles, operation and construction of vacuum diodes, and in particular their use in thermionic generators and the similarly constructed vacuum diode heat pump, as well as the principles and prior art of thin film carbonaceous coating.